Does the wall appear to be scalable? Is it made of stone or wood? I'm just thinking of other ways Sar'ai could try to over the wall, although, strength is not her strong suit…
You're not in a good position to just pour it wherever you want, especially with arrows hitting your cover on a constant basis.
The safe bet would be to try and throw the casks down to shatter where you think the splash pattern will do the most good.
You could lean out in order to try and get the liquid specifically where you want, but that will expose you to quite a bit of archer fire.
Brandon, the outer portion of the wall is composed of vertical logs. Scaling it will not be simple or swift, but it can be done. In the meantime though, you'll be exposed, either from behind or above, depending on who thinks you're their enemy.